Subatomic particle detector with liquid electron multiplication medium

ABSTRACT

A subatomic particle detector having a large number of equally spaced anode conductors arranged in a single plane opposite and parallel to a large cathode plate with the space between the anode conductors and cathode plate filled with liquid argon. A phototransistor is connected to each conductor for automatic readout of the detector by means of a laser beam that is scanned over each phototransistor.

United States Patent Alvarez et al.

[15] 3,659,105 51 Apr. 25, 1972 [54] SUBATOMIC PARTICLE DETECTOR WITHLIQUID ELECTRON MULTIPLICATION MEDIUM Inventors:

Assignee:

Filed:

Appl. No.1

U.S.Cl......

Int. Cl.

Luis W. Alvarez; Stephen E. Derenzo; Richard A. Muller, all of Berkeley;Robert G. Smits, Lafayette; llaim Zaklad, Berkeley, all of Calif.

The United States of America as represented by the United States AtomicEnergy Commission Oct. 21, 1970 ..250/83.6 R, 250/7l.5 R, 250/831,

................................ ..G01t 1/18, HOlj 39/26 Field of Search..250/83.6 R,7l.5 R, 83.3 R,

[56] References Cited UNITED STATES PATENTS 3,337,734 8/1967 Chubb..250/83.6 R

Primary Examiner-Morton J. Frome Attorney-Roland A. Anderson [57]ABSTRACT 11 Claims, 7 Drawing Figures PATENTEDAPR 25 m2 SHEET 20F 2 57 f54 53 LASER 0 55 v 58 a; %%S DATA 4 PROCESSING 59 4g SYSTEM 6l :251? feeDISC. I

level 'llllfll-llllmllm INVENTORS LUIS W. ALv/JfiEZ STEPHEN E. DERE/VZORIC/M RD A. MULLER ROBERT 6. 544/759 HAIM ZAKLAD ATTORNEY:/5-( m 4.1

SUBATOMIC PARTICLE DETECTOR WITH LIQUID ELECTRON MULTIPLICATION MEDIUMBACKGROUND OF INVENTION The present invention relates to subatomicparticle detectors and more particularly it relates to a filmlessdetector that provides high spatial resolution and automatic readout.

A widely used filmless subatomic particle detector which is adaptable toautomatic readout is the gas-filled wire chamber. These chambers,however, lack the spatial resolution necessary to interpret the resultsof experiments being carried out at increasingly higher energies. Ithasbeen found that regardless of how close the wires in such a chamberare spaced, the center of a track in the chamber cannot be determinedwith an rms error ofless than e= 0.2 tan 0.2, where e is in mm and 6 isthe angle of the track with respect to the chamber normal.

SUMMARY OF INVENTION In brief, the present invention pertains to asubatomic particle detector that has a cathode having a large surfacearea separated a very small distance from an anode having a relativelysmall surface area, with the space between the anode and cathode filledwith a liquid having high electron mobility as an electronmultiplication medium.

It is an object of the invention to detect the paths of subatomicparticles with a high degree of spatial resolution and to automaticallyreadout the results of the detection.

Another object is to use a liquid having high electron mobility as anelectron multiplication medium in a particle detector.

Another object is to eliminate spark discharges in a particle detectorhaving closely spaced electrodes.

Other objects and advantageous features of the invention will beapparent in a description of a specific embodiment thereof, given by wayof example only, to enable one skilled in the art to readily practicethe invention, and described hereinafter with reference to theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a cross-sectional view of aliquid-filled ionization chamber with typical electrical connections,according to the invention.

FIG. 2 is a typical waveform of an output signal from the chamber ofFIG. 1.

FIG. 3 is a schematic diagram of a multiwire particle detector includinga readout system using a phototransistor connected to each wire and alaser beam for scanning the phototransistors.

FIG. 4 is a pulse diagram showing the difference in height betweenpulses from the phototransistors of FIG. 3 under normal conditions andphototransistors connected to wires adjacent to an ionized region.

FIG. 5 is an expanded partial end view of a first construction of ananode for the detector of FIG. 3.

FIG. 6 is an expanded partial end view of a second'construction of ananode for the detector of FIG. 3.

FIG. 7 is a partial top view of a third construction of an anode for thedetector of FIG. 3.

DESCRIPTION OF AN EMBODIMENT Referring to the drawing there is shown inFIG. 1 a detector 20 for detecting subatomic particles. The detector 20is comprised of glass tubing 22 with an elongated conductor in the formof a fine electrically conducting wire anode 23 stretched between theends of the tubing. A portion of the wire anode extends through and iscoaxial with a cylindrical cathode 25 which is situated on the innersurface of the glass tubing and is electrically connected over ahigh-voltage lead 26 to the negative pole of a voltage source 27. Theupper end of the wire anode is connected over a lead 29 to anoscilloscope 30 having an input impedance 32. The total parasiticcapacitance of the detector 20 is indicated as a capacitance 33. Thetubing 22 defines a chamber 35 which is filled with a noble gas such asxenon or argon in the gaseous state through a filling arm 38. The tubing22 is mounted in a refrigerated chamber (not shown) so that the noblegas is condensed to provide a liquid 36 in the tubing.

In operation, the detector is placed in the path of a radioactivesource. Passage of a particle through the chamber 35 in the spacebetween the anode and cathode ionizes the liquid gas atoms along thepath of the particle. The voltage applied to the cathode 25 ismaintained at a level that will supply the free electrons resulting fromthe ionization with sufficient energy to make inelastic collisions withthe liquid gas atoms and to cause electron multiplication by anexponential avalanche of electrons toward the anode. However, thevoltage applied across the anode and cathode is maintained below thelevel that would cause field emission from the cathode to anode and leadto a spark discharge. Typically, for a detector using liquid argon andhaving a fine wire anode from 4-20 pm in diameter and separated 2 mmfrom the cathode, the counting threshold was found to be around 5,200 Vand the spark discharge threshold to be around 6,200 V. As a result ofthe avalanche a pulse is developed on the lead 29; atypical 50pico-coulomb pulse 39 is shown in FIG. 2. Pulse height, however, hasbeen found to be sensitive to the liquid pressure and tends to decreasewith an increase in pressure.

In order to obtain electron multiplication in a liquid, it has beenfound necessary to maintain a high degree of electron mobility which ingeneral is inhibited by the presence of impurities in the liquid. Inliquid argon for example, the presence of liquid oxygen as an impuritytends to capture the free electrons. The purity necessary for liquidargon has been found to be less than 1 part of oxygen impurities permillion parts of argon. This degree of purity for liquid argon has beenobtained by condensing gaseous argon from a supply that has beenpurified by passing it through an active metal getter. High electronmobility is essential for avalanche multiplication and is known to bepresent only in one class of liquids: the noble gases.

Referring to FIG. 3, there is shown an example of a multianode arrayparticle detector 41 suitable for determining the path of an ionizingparticle with a high degree of resolution. The detector 41 is comprisedof a plurality elongated conductors in the form of fine anode wires 42mounted on a flat support 44 with a cathode plate 45 spaced from thewires and arranged to be parallel thereto. The space between the wiresand plate is filled with a liquid having high electron mobility. Anegative pole 47 of a voltage source is connected to the cathode plate45.

Upon passage of a particle through the detector, the liquid is ionizedalong the path of the particle and a charge induced in adjacent wires42. In order to readout and record the particle path, each of the wiresmay be successively connected to a data processing system 48. Suchconnection may be by means of successively actuated electronic switchessuch as used in the system shown in FIG. 3 wherein a phototransistor 50is connected to one end of each of the wires 42. The emitter of eachphototransistor 50 is connected to the associated wire 42 while thecollectors are connected together in common to the input of an amplifier51. The phototransistors 50 are available in arrays on integratedcircuit chips which may be arranged on the support 44 with thephotosensitive laser exposed for scanning by a laser beam 53 from alaser 54. The beam 53 is rotated over the transistors by means of amultifaced reflector 55.which is rotated by a motor 57. As the beamfalls on each phototransistor base, the phototransistor is biased toconduct a charge on the associated wire to the amplifier 51. Each wireadjacent the path of the particle has been charged by an electronavalanche and therefore conducts a, relatively large charge through theassociated phototransistor to the amplifier, resulting in a relativelylarge pulse 61 (FIG. 4) at the amplifier output. Each of thephototransistors not associated with a charge wire develops a low levelpulse 62 at the amplifier output. The pulses 62 are due to a smallphotovoltaic emf induced in each phototransistor upon its base beingexposed to the laser beam.

The pulses at the output of discriminator 58 areapplied to the dataprocessing system 48 for indexing the system with respect to thescanning of the wires 42. The pulses at the output of the discriminator59 are also applied to the system 48. A coincidence of pulses fromdiscriminators 58 and 59 indicates the position of a charged wire withrespect to the scanning of wires 42. The system 48 includes storagemeans that stores this information for indicating the position of theparticle path. In a practical particle detection system a second anodeand cathode (not shown) would be required to fully define the particlepath. The second anode and cathode would have to separate connection tothe system 48 and would be oriented parallel to the first anode andcathode with the two sets of anode conductors at right angles.

Various arrangements of elongated conductors that may be used as theanode are shown in. FIG. 5, 6, and 7. In FIG. 5a partial end view of thewire conductors 42 are shown mounted on the support 44. In FIG. 6,half-cylindrical conductors 65 are shown bonded to a support 66. It isconvenient to fabricate the half-cylindrical conductors 65 by firstplating thin strips of metal on the support 66- using thin filmtechnique and then plating more metal on the strips to obtain thehalf-cylindrical form. In FIG. 7 elongated conductors68 are shownmounted on a support 69. A multitude of protrusions 71 extend from eachof the conductors 68. Conveniently, the protrusions may be in the formof pyramids having square bases. Such conductors have been developed byStanford Research Institute, Menlo Park, California and have beenfabricated to have pyramids l/ l ,000 inch high on H1 ,000 inch centers.

It has been found that a liquid particle detector with a very smoothcathode and a large number of small protrusions extending from the anodeenhance the sensitivity of the detector to produce electronmultiplication without sparking. This is due to electric fieldconcentrations obtained at the tips of the protrusions. The sensitivityof any particular anode-cathode configuration to electron multiplicationmay be referred to as an enhancement factor which may be defined as theratio of the field emission threshold voltage with a negative potentialapplied to the cathode to the field emission threshold voltage with anegative potential applied to the anode. For detectors using purifiedliquid argon, enhancement factors of at least 110 have been found to berequired for practical detector operation. Thus, when using the wires 42and half-cylinders 65, it is desirable to give their surface a fineroughness such as by etching to provide a large number of fieldconcentration points. With the conductors 68, the pyramids 71 providefield concentrations points whose'location and height are preciselycontrolled.

For a more complete discussion of the development and theoreticalaspects of the invention reference is made to a U.S. Atomic EnergyCommission technical report No.

UCRL-19254 The Prospect of High Spatial Resolution For CounterExperiments: A New Particle Detector Using Electron Multiplication inLiquid Argon," by Derenzo, Muller, Smits and Alvarez.

While an embodiment of the invention has been shown and described,further embodiments or combinations of those described herein will beapparent to those skilled in the art without departing from the spiritof the invention.

We claim: 1. A particle detector, comprising: an anode; a cathode spacedfrom said anode and having a substantially larger surface area than saidanode; a liquid filling the space between anode and cathode, said liquidhaving high electron mobility; and means for applying a voltage acrosssaid anode and cathode, said voltage being applied to hold said anodepositive with respect to said cathode, the level of said voltage andrelative surface areas of said cathode and anode being arran ed to givea field strength between said anode and ca ode that will sustain anionization avalanche in said 5. The particle detector of claim 4,wherein each of saidprotrusions is a pyramid having a square base.

6. The particle detector of claim l,'wherein said anode is comprised ofa plurality of elongated conductors and said cathode is a conductorhaving a continuous surface.

7. The particle detector of claim 6, wherein said conductors are finewires.

8. The particle detector of claim 6, wherein said conductors aresemi-cylindrical and are integral with a dielectric sheet.

9. The particle detector of claim 6, further including:

data processing means; and

means for successively connecting each of said elongated conductors tosaid data processing means to record the path of said ionizing particle.

10. The particle detector of claim 6, further including:

light responsive means connected to said plurality of conductors;

means for scanning a light beam over said light responsive means forgenerating first signals in said light responsive means that correspondto said conductors being adjacent a normal region of said liquid and forgenerating second signals in said light responsive means that aredifferent from said first signals and which correspond to saidconductors 'being adjacent an ionized region of said liquid; and

means responsive to said first and second signals for storing andindicating the position of said conductors adjacent an ionized region ofsaid liquid.

11. The particle detector of claim 1, wherein said liquid is xenon.

1. A particle detector, comprising: an anode; a cathode spaced from saidanode and having a substantially larger surface area than said anode; aliquid filling the space between anode and cathode, said liquid havinghigh electron mobility; and means for applying a voltage across saidanode and cathode, said voltage being applied to hold said anodepositive with respect to said cathode, the level of said voltage andrelative surface areas of said cathode and anode being arranged to givea field strength between said anode and cathode that will sustain anionization avalanche in said liquid in response to passage of anionizing particle through said liquid, which field is below the strengthrequired for field emission from said cathode to said anode.
 2. Theparticle detector of claim 1, wherein said liquid is a liquid noble gas.3. The particle detector of claim 2, wherein said liquid is argon andhas a purity of less than 1 part of impurities per million parts ofargon.
 4. The particle detector of claim 1, wherein said anode is anelongated electrical conductor having a multitude of electricallyconducting protrusions extending therefrom.
 5. The particle detector ofclaim 4, wherein each of said protrusions is a pyramid having a squarebase.
 6. The particle detector of claim 1, wherein said anode iscomprised of a plurality of elongated conductors and said cathode is aconductor having a continuous surface.
 7. The particle detector of claim6, wherein said conductors are fine wires.
 8. The particle detector ofclaim 6, wherein said conductors are semi-cylindrical and are integralwith a dielectric sheet.
 9. The particle detector of claim 6, furtherincluding: data processing means; and means for successively connectingeach of said elongated conductors to said data processing means torecord the path of said ionizing particle.
 10. The particle detector ofclaim 6, further including: light responsive means connected to saidplurality of conductors; means for scanning a light beam over said lightresponsive means for generating first signals in said light responsivemeans that correspond to said conductors being adjacent a normal regionof said liquid and for generating second signals in said lightresponsive means that are different from said first signals and whichcorrespond to said conductors being adjacent an ionized region of saidliquid; and means responsive to said first and second signals forstoring and indicating the position of said conductors adjacent anionized region of said liquid.
 11. The particle detector of claim 1,wherein said liquid is xenon.